Understanding Standard English Machine Screw Sizes

Machine screws are one of the most widely used products among standard fastener types. Despite their prevalence, there is no strict definition of a machine screw, and the term covers a broad range of fastener varieties. Machine screws are available in different models, sizes, materials, and configurations. They are usually smaller, blunt-ended, fully threaded, and stronger than other screw types. The American machine screw standard was adopted in 1949 and includes a fine thread (UNF) and a coarse thread (UNC). Machine screw sizes are described using numbers and letters, with the first number expressing the diameter and the second indicating the number of threads per inch (TPI).

English Machine Screw Characteristics and Values

Machine screw types

Machine screws are one of the more widely used products among standard fastener types. Despite their prevalence, there is no strict definition of a machine screw, and the term covers a broad range of fastener varieties. Machine screws are available in different models, sizes, materials, and configurations.

Stainless Steel Machine Screws

Machine screws made of stainless steel are a common variety. They are known for their strength and resistance to corrosion.

Brass Machine Screws

Brass machine screws are another option. Brass is a soft metal with a gold colour that can be polished to a shiny finish. Brass screws are often chosen for their appearance and used in decorative applications.

Plated Machine Screws

Plated machine screws are those that have been coated with a layer of material, such as zinc or chrome, to improve their appearance, corrosion resistance, or other properties.

Slotted or Flat-Head Machine Screws

Flat-head machine screws, also known as countersunk screws, have a flat top and a countersunk underside, allowing them to sit flush with the surface they are driven into. They are often chosen for their neat and flush finish.

Phillips Head Machine Screws

Phillips head machine screws feature a cross-shaped recess in the head and are designed to be used with a Phillips screwdriver. They offer more torque than a standard slot drive.

Torx Head and Hex Head Machine Screws

Torx head machine screws, also known as star drive, have a six-pointed star-shaped recess and require a special Torx driver for installation. Hex head machine screws, on the other hand, have a hexagonal head and can be installed using a wrench or a socket.

Machine screw uses

Machine screws are used to join two or more objects together. They are typically used for fastening metal parts securely, although they can be used for a wide range of applications. Machine screws are often used in the assembly of electronic devices, automotive repair, plumbing repairs, window installation, metalworking projects, door hinge installation, and cabinet installations. They can also be used for furniture assembly, roofing repairs, and in masonry projects.

Machine screws are ideal for projects that require strength and stability. They are made from thick metal, usually steel or brass, and have pitched threads that allow them to screw into tapped holes easily. They are often used in heavy-duty or industrial-scale assembly and construction jobs.

Machine screws are also useful when working in tight spaces as their size allows them to fit where other fasteners may not. They can be used to join different parts or components and are often chosen to hold down or sandwich gaskets and membranes. They can also be used to separate and secure parts or panels that need to be kept at a fixed distance from one another. This can be achieved by using a threaded coupling, allowing two machine screws to be driven in from opposite directions.

Machine screws are available in a variety of sizes, head types, materials, and thread gradings. Common head types include hex head, flat head, oval head, and cheese head. They can be made from stainless steel, brass, or aluminium. Stainless steel is a popular choice for its strength, durability, and rust resistance.

Machine screw measurements

Machine screws are typically smaller in length and diameter than other types of bolts and fasteners. They are almost always blunt-ended, and most are fully threaded. There are a variety of different models, sizes, materials, and configurations of machine screw available.

A common standard for measuring screw diameter is the metric scale, where the letter M is followed by a number. The higher the number, the larger the diameter. For example, M2.5, M3, M3.5, M4, and M5. In this example, the number after the M denotes the nominal outer diameter of the screw in millimetres.

US machine screw dimensions are described using numbers such as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32, 10-32, and 10-24. These numbers represent the diameter and the number of threads per inch (TPI). For instance, a #10 screw has a diameter of 0.190 inches or 4.826 mm.

Machine screw materials

Machine screws are made from a variety of materials, including steel, brass, aluminium alloy, and stainless steel. The choice of material depends on the specific application and requirements.

Aluminium is a ductile, non-magnetic, corrosion-resistant, and low-density element. It is commonly used in the transportation, aerospace, and construction industries due to its strength and lightweight properties.

Brass, an alloy composed mainly of copper and zinc, offers low friction, excellent workability, durability, and a non-sparking nature. It is often used for plumbing, architectural, and decorative applications, as well as for nuts, bolts, washers, and injectors.

Steel alloys, primarily made of iron and carbon, are known for their high tensile strength. They find applications in various industries, such as transportation and defence. Stainless steel, a type of steel alloy containing at least 10.5% chromium, is renowned for its strength, corrosion resistance, stain resistance, and ease of sterilisation. It is widely used in construction, manufacturing, medical equipment, and household items.

Titanium, a transition metal, is exceptionally strong, lightweight, and corrosion-resistant. It is commonly used in the aerospace, automotive, sanitary, and medical fields.

Machine screw head types

Machine screws are available in a variety of head types, each serving a specific functional or decorative purpose. The two primary screw head designs are countersunk and non-countersunk.

Non-Countersunk Head Types

Non-countersunk screw heads are fully exposed and come in a wide range of varieties. Some of the most common types include:

• Binding head screws, also known as binder head screws.
• Button head screws have a rounded, low-profile head with a hexagonal socket. They are ideal for confined spaces and light-duty applications like removable panels.
• Cheese head screws feature cylindrical sides and a flat disc top with a flat bearing surface.
• Fillister head screws, also known as raised cheese head screws, have a large, oversized head and are preferred for counterbored holes.
• Flange head screws have a flange diameter and height designed to fit a variety of applications and counterbored holes.
• Hex head screws, also known as hex bolts, have a six-sided hexagonal head and an externally threaded body. They are typically used with a washer and hex nut assembly or a tapped hole.
• Pan head screws have a wide head, a flat bearing surface, high vertical sides, and a flat or slightly domed top. They are a common type of non-countersunk screw head used in machine screws.
• Round head screws.
• Socket head screws have a cylindrical barrel-shaped head with a hexagonal socket. They are well-suited for confined spaces as they allow for reduced component size and material costs.
• Square head screws.
• Truss head screws.

Additionally, some screw head designs combine features from multiple types, such as the slotted hex, hex washer, slotted hex washer, and round washer head screws.

Countersunk Head Types

Countersunk screw heads are designed to sit flush with the surface of the material, with little to no protrusion. Common types of countersunk screw heads include:

• Flat head screws are ideal for finish work and can be used with a flat head screw cover, also known as a "beauty cap," to hide the head.
• Oval head screws typically require a countersunk hole unless the material is very soft.
• Bugle head screws are commonly found in drywall screws. The head design automatically compresses the drywall paper and gypsum during installation, forming its own countersunk hole.

Drive Styles

The drive style, or drive recess, refers to the type of tool required to install or remove the screw. Common drive styles include:

• Slotted screws, which use a flat-bladed or slotted screwdriver.
• Phillips screws, which use a Phillips screwdriver with a cross-shaped head.
• Hex screws, which require a hex key, also known as an Allen key.
• Torx screws, which use a star-shaped Torx driver.
• Square screws, which use a square driver.
• Combination screws, which can use either a slotted or Phillips screwdriver.

Frequently asked questions